Is it ensure, insure, or assure?
Old English ferian "to carry, convey, bring, transport," from Proto-Germanic *farjanan (cf. Old Frisian feria "carry, transport," Old Norse ferja "to pass over, to ferry," Gothic farjan "travel by boat"), from PIE *per- "going, passage." Related to fare (v.). Related: Ferried; ferries; ferrying.
early 15c., "a passage over a river," from Old Norse ferju- "passage across water," ultimately from the same Germanic root as ferry (v.). The modern noun (1580s) is a shortening of ferry boat (mid-15c.).